So why do they have a huge Wh/km advantage over rivals such as Porsche? https://www.xautoworld.com/opinion/taycan-201-tesla-range/

Less aggressive regenerative breaking to compensate for cooling deficiency, and less efficient inverter.

Tesla being able to "turn off" front motor, and lack of differential probably also adds a little bit to efficiency.

I thought switched reluctance was about cost reduction (with maybe a few percent efficiency increase). Maybe weight too.

It's more efficient than an induction motor. An induction motor has the most torque when it's at rest but then generates its own resistance as it spins up and so loses efficiency. In the dual motor setup, Tesla use both types to get the best of both worlds: induction to accelerate and then it cuts off at some point and uses only the switched reluctance for efficiency.

Man, induction motor has 0 torque when at rest

My mistake. It has the most torque when it first starts spinning.